There are numerous types of unwanted vibrations, noises, pulsations, disturbances, and other forms of fluctuating vibratory energy that can exist in a vehicle; these phenomena are hereafter collectively and broadly referred to as ‘vibrations’. Vibrations can have many sources, including external sources such as irregular road surfaces, as well as internal sources.
An example of an internal source of vibrations is a non-concentric, out-of-round, or otherwise irregular rotating part. For instance, if a tire, wheel, hub and/or rotor is manufactured or mounted to the vehicle in a non-concentric or off-balance manner, then the part rotates with an uneven weight distribution. This, in turn, can produce periodic or harmonic vibrations in the vehicle; that is, vibrations having a first order component centered at a first order frequency, as well as higher- or multi-order components centered at frequencies that are integer multiples of the first order frequency. A first order component of a periodic vibration is generally centered at the same frequency as the rotating object from which it emanates and usually has a greater amplitude or intensity than its higher-order counterparts. For example, a wheel rotating at two rotations per second (2 Hz) can produce periodic vibrations having a first order component at 2 Hz, a second order component at 4 Hz, a third order component at 6 Hz, and so on. The first order or 2 Hz component is usually more intense than the second and third order components. It should be appreciated that non-concentric rotating parts are only one potential source of periodic vibrations in a vehicle, as many others also exist.
Periodic vibrations caused by internal sources can propagate throughout the vehicle and can cause an undesirable shake or movement of certain vehicle components that is noticeable to the driver. For instance, periodic vibrations generated at the wheel assemblies or corners can combine to create a dynamic torque on a steering wheel column component that causes the steering wheel to turn in either direction. When this type of event occurs on a flat or smooth road surface, it is all the more apparent to the driver and is sometimes referred to as ‘smooth road shake’ (SRS) or torsional nibble.
A variety of techniques for reducing or mitigating periodic vibrations in the vehicle have been employed. These techniques include on- and off-vehicle wheel balancing, using different types of damping components, and attempting to machine or otherwise produce more concentric and precise parts.